The buried army of emperor Qin
Xi'an was the capital of the Qin (pronounced T'chin) dynasty. The last Emperor of this dynasty, Qin Shi Huangdi, conquered the 6 neighboring kingdoms and unified China in 221 B.C.
Fearing death and his passage into the afterlife, he ordered, since his accession to the throne at the age of 13 years, over 500,000 people, involving many trades, to work on his tomb by creating thousands terracotta warriors ! The work lasted for 36 years until his death.
In addition to it, the tomb of Qin, a little away from the buried army and under a mound would consist of an arch and sparkling gems in a sea of mercury ! on which the sarcophagus of the Emperor would float...
The tomb is still unexplored, partly because of the presence of mercury, as emanation of it in the atmosphere could have bad consequences...